There are a number of inductance measurement devices for sensing the position or displacement of a movable member in an inductor. Such inductive displacement sensors normally comprise a magnetic coil containing a movable iron core. The inductance value of the coil depends on the depth of immersion of the core. Therefore, the inductance value is a measure for the displacement of the iron core.
Further, there are a number of electronic circuits which function on the basis of different operating principles for measuring or evaluating the inductance of inductive displacement sensors. For example, published German Patent Applications DE-OS 35 19 978 and DE-OS 33 43 885, the latter of which corresponds to U.S. Pat. No. 4,649,341, show and disclose several types of control circuits for inductive sensors in which a voltage pulse is applied to the inductive coil. This current through the coil then rises from a zero level to a value determined by a current limit. The time period from the beginning of the current rise until the actuation of the current limit is measured with the aid of a suitable microcomputer, which serves as a convenient and efficient means for measuring the inductance of the coil which is used to determine the position of the iron core.
These prior art circuits have the disadvantage that a rise in the ambient temperature in the vicinity of the sensor results in an increase of the resistance of the coil as well as in an increase of the permeability of the movable iron core. Since the two factors cumulatively lead to an increase in the charging time for the inductance, an error occurs in the measuring of the displacement or position of the iron core. A measuring error is particularly serious when the displacement sensor is used in a motor vehicle in which ambient temperatures may vary between -40.degree. to +80.degree. C.
Previously, protective circuts for the sensor coils were included in a negative temperature coefficient (NTC) thermistor network, which is capacitor of compensating for the temperature variations as shown and disclosed in published German Patent Application DE-OS 35 26 560. However, an (NTC) resistance network of this kind leads to additional expense, which is particularly disadvantageous with mass produced components, such as those required for motor vehicles.